Backport of:

-----------------------------------------------------------
2630.28.28 Magne Mahre  2008-12-05
Bug #38661 'all threads hang in "opening tables" or "waiting for table"
            and cpu is at 100%'
                      
Concurrent execution of FLUSH TABLES statement and at least two statements
using the same table might have led to live-lock which caused all three
connections to stall and hog 100% of CPU.
        
tdc_wait_for_old_versions() wrongly assumed that there cannot be a share
with an old version and no used TABLE instances and thus was failing to
perform wait in situation when such old share was cached in MDL subsystem
thanks to a still active metadata lock on the table. So it might have
happened that two or more connections simultaneously executing statements
which involve table being flushed managed to prevent each other from
waiting in this function by keeping shared metadata lock on the table 
constantly active (i.e. one of the statements managed to take/hold this
lock while other statements were calling tdc_wait_for_old_versions()).
Thus they were forcing each other to loop infinitely in open_tables() - 
close_thread_tables_for_reopen() - tdc_wait_for_old_versions() cycle
causing CPU hogging.
        
This patch fixes this problem by removing this false assumption from
tdc_wait_for_old_versions().
 
Note that the problem is specific only for server versions >= 6.0.
        
No test case is submitted for this test, as the test infrastructure
hasn't got the necessary primitives to test the behaviour.  The
manifestation is that throughput will decrease to a low level
(possibly 0) after some time, and stay at that level. Several
transactions will not complete. 
        
Manual testing can be done by running the code submitted by Shane 
Bester attached to the bug report.  If the bug persists, the 
transaction thruput will almost immediately drop to near zero 
(shown as the transaction count output from the test program staying 
on a close to constant value, instead of increasing rapidly).

sql/sql_base.cc

@@ -8324,8 +8324,7 @@ tdc_wait_for_old_versions(THD *thd, MDL_request_list *mdl_requests)
     {
       if ((share= get_cached_table_share(mdl_request->key.db_name(),
                                          mdl_request->key.name())) &&
-          share->version != refresh_version &&
-          !share->used_tables.is_empty())
+          share->version != refresh_version)
         break;
     }
     if (!mdl_request)